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Requirements Confusion

Over on Seilevel's Requirements Defined message board, Joy Beatty wrote about her recent experience at a requirements conference:

I heard some new ideas about NFRs [nonfunctional requirements] - One was that the distinguisher is that FRs [functional requirements] are quantifiable and NFRs are not. I'm not sure that I think any requirement is not quantifiable if you try hard enough to make it so.

A further explained response was that using “testable” as a distinguisher, meant that the requirement can be tested by execution of the system (FR = function or performance) vs something you might test about the environment of the system (NFR = maintainability).
In my opinion, these anecdotes only underscore just how misunderstood requirements are. People at a requirements conference were saying these things? Wow.

As I've mentioned, a nonfunctional requirement is just as testable and quantifiable as a functional requirement.

Functional requirements state what the system should do. The system either does it or doesn't do it. So sure, functional requirements are testable.

Nonfunctional requirements are simply metrics that we attach to functional requirements. While the system is doing whatever it's supposed to be doing, we can measure things like throughput (performance), how long it takes for a typical user to accomplish her goals (usability), the percentage of time the system is available to deliver any particular functionality (availability), etc. These nonfunctional requirements are by their very nature testable (at least in principle) and quantifiable.


Kevin Brennan said…
Absolutely. I'm not sure there is such a thing as a non-testable requirement; if you can't define a way to test it how can you state whether or not it has been met?

A non-testable requirement might be more accurately described as a goal.

Quantifiable is also not a good description--I can easily think of functional requirements that aren't quantifiable.
Joy said…
Roger, I'm with you on this, I was just quoting what I heard. I can't comprehend writing a requirement that you cannot measure.

Though I will slightly argue with Kevin's comment that quantifiable may actually not be an ok term - it may just be semantics though. But, right or wrong, I equated in my mind that "quantifiable" is the same as "measurable".
Kevin Brennan said…
Hi Joy,

Is a requirement like "the user interface will be available in English, French, and Spanish" quantifiable, in the way you use the term? (It's certainly testable). I don't think of it that way, but if you do then our disagreement is probably just semantic.

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